1. Field of the Invention
The present invention relates to the oxidation of aromatic compounds bearing an oxidizable alkyl group, notably for the preparation of aromatic ketones or aldehydes.
This invention especially relates to the oxidation of aromatic compounds bearing an oxidizable alkyl group and a hydroxyl and/or alkoxy group, notably for the preparation of hydroxy and/or alkoxy-aromatic ketones or aldehydes.
In a preferred embodiment of the present invention, cresols are oxidized into corresponding hydroxybenzaldehydes.
2. Description of the Prior Art
EP-A-12,939, U.S. Pat. No. 4,453,016 and U.S. Pat. No. 4,471,140 describe the oxidization of p-cresol by means of molecular oxygen in the presence of sodium or potassium hydroxide and a catalyst, such as a compound of cobalt, nickel or chromium, to produce the alkali metal salts of p-hydroxybenzaldehyde.
A preferred embodiment entails using methanol as the reaction solvent and cobalt salts as the catalyst.
Such processes employ a large excess of potassium or sodium hydroxide, providing highly concentrated and viscous reaction media. Moreover, in order to separate the p-hydroxybenzaldehyde formed, it is necessary to neutralize the excess of base, which promotes the formation of large amounts of salts.
To obviate these disadvantages, U.S. Pat. No. 4,748,278 describes a process to isolate the p-hydroxybenzaldehyde formed, but this entails supplementary stages.
Moreover, JP-A-63/154,644 describes a process for the oxidation of p-cresol by means of oxygen, in the presence of a catalyst formed from cobalt acetate, cobalt bromide and manganese acetate. The reaction is conducted in a mixture of acetic acid and acetic anhydride, because it is necessary to protect the hydroxyl group by transforming it into an acetoxy group.
Therefore, such process suffers from the disadvantage of requiring the protection of the hydroxyl group. Too, the reaction is not aldehyde selective, because more acid forms, the yield of the acetoxybenzoic acid obtained being approximately 80%.